Transformers for distribution and power have improved greatly in the last decade due to improved materials, and sophisticated design tools for optimizing performance, cost and size. Recent energy saving legislation in North America commonly known as “Energy Star” in the USA and “C 802” in Canada drive the issues of cost and energy savings, which has spawned significant developments in the art of transformer design. Manufactures are faced with an ever-increasing competitive market and stringent power efficiency requirements for their products.
A large part of transformer costs is based on material, such as the copper/aluminum (for windings) and steel (for magnetic cores). Magnetic materials available to the transformer industry have been designed for known transformer topologies. The producers of ‘soft magnetic materials’ for the transformer industry, have consequently, made it difficult to realize new transformer topologies.
Transformers can take many forms. Some are applied to single phase or three phase applications and others provide a multitude of voltages and phases depending on the need and application. Known transformer topologies can take various forms, for example the most common single or 3-phase transformers are classified as ‘core type’ or ‘shell type’ transformers. The core type transformer is recognizable by external windings surrounding a magnetic core, whereas a shell type transformer is recognized by a core extending around a part of the windings.
Transformer size dictates the power handling capacity of the transformer and its ability to dissipate transformer generated heat produced as a result of transformer energy or power losses. Usually, the two greatest loss components are contributed by the resistive losses in the transformer, hysteresis and eddy current loss in the core. A cooling mechanism is needed to dissipate the heat maintaining a thermal equilibrium of the transformer, as otherwise “thermal runaway” occurs and the transformer fails.
Thermal runaway occurs when the energy or power losses of the transformer produces more heat than can be dissipated by the transformer. The ability to dissipate heat of a transformer is a function of many things, including: thermal resistance of the windings/core to a cooling medium (e.g. oil or air), a dissipation constant, a thermal coefficient of resistance of windings, core properties, a thermal resistance of an electrical insulation system used to electrically insulate the windings, a physical geometry, and enclosure type, if used. Transformers most commonly used in the power and distribution industry are of ‘dry type’, i.e. where air is used as the cooling medium. As such, cooling of these transformers is predominantly performed by air passing around the windings.
For this reason, prior art transformer designs include portions of the windings and/or parts of the magnetic core that protrude or are exposed to the surrounding air (or other medium). This exposure to the medium permits the required cooling to prevent thermal runaway, and also compensates for an imperfect optimization between steel and copper content within available magnetic laminations or strip steel assembly configurations. In dry type transformers, the windings are normally configured to allow air to flow between the winding layers thus effectively increasing the cooling surface area. This is very wasteful in terms of winding wire material content since winding wire is expensive and can contribute to over half the total material content. Also the exposure of the windings and core brings about external leakage of flux. Furthermore, the thermal transfer between the copper winding and air is best when the winding is directly exposed to the air, but cannot exceed a certain thermal transfer rate. Typically 20 uW per mm2 per degree Centigrade rise.
In reality the minimum material content of transformers are not materialized because of the thermal dissipation requirements, and because the costs of materials, practical constraints on construction methods, etc. The toroidal transformer, which has the characteristics of minimizing materials and magnetic leakage losses, is generally the most optimum core type transformer design currently available. However, toroidal transformers cannot be easily configured into 3-phase transformers where portions of the core can share and partially cancel magnetic flux vectors.
The technical challenge in designing transformers is only exacerbated with increase in power losses due to the winding current. Larger power transformers produce more heat. The relationship between dissipation and temperature rise as a function of transformer dissipating surface area is not a linear function, and below a certain critical surface area, losses and temperature rise vs. winding current increase exponentially. This critical surface area is a constraint on the size of the transformer. Furthermore, as cores get larger the ratio of surface area to volume of material decreases, thus the capacity to dissipate heat becomes more of a problem for a certain dissipation per cubic meter. In high power transformers cores can be large enough to cause very high temperature rises inside the core causing dimensional distortion and mechanical stresses that affect magnetic properties of the core. Also, for very large transformers, the core heat affects the winding adjacent to the core requiring extra spacing to cool the core and winding. This further decreases efficiency of the transformer, and increases material costs, noise and vibration of the transformer.
Accordingly, a topology for a transformer is required that can reduce material costs, improve efficiency, or provide a compact arrangement with acceptable thermal dissipation for a given power requirement.